Walnut twig beetle (WTB), Pityophthorus juglandis, is a small native phloeophagous (phloem-feeding) insect that has recently been associated with the fungus Geosmithia morbida. This fungus and WTB are the principal agents involved in thousand cankers disease (TCD). This disease is fatal to walnut trees and is responsible for the gradual decline of several species of black walnut in the western United States during the past decade. Walnut and butternut appear to be the only hosts.
The disease has spread widely in the western United States and has now been detected in eastern states—including Tennessee (June 2010), Virginia (May 2011), and Pennsylvania (August 2011)—threatening the highly valuable native timber stands of eastern black walnut, Juglans nigra. Populations of WTB have been invariably associated with the fungus: this type of dieback of walnut has been found only where the beetle is present. Thus, capturing and identifying the minute WTB is the key to early detection of TCD in new areas
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